For decades, a groundbreaking brain study fueled opinions about whether we have free will over our actions. The conclusions drawn from this experiment, however, were likely flawed.
A brief history of the neurobiology of free will: discovering the Bereitschaftspotential
In 1964, two scientists at the University of Freiburg in Germany monitored the electrical activity of people’s brains during self-initiated movements (finger flexions). Each day over several months, participants sat in a chair, had wires affixed to their scalp via a special recording device called an electroencephalogram (EEG), and were instructed to flex a finger on their right hand at intervals that pleased them, up to 500 times per visit.
Drs. Hans Helmut Kornhuber and Lüder Deecke were interested in looking at brain activity related to volitional acts, or the decision to act. Researchers used a computer to measure participants’ brain activity from the motor cortex and supplementary motor area (parts of the brain that control movement) while participants flexed their finger. They found that in the milliseconds leading up to each finger flexion, there was a gradual increase in brain activity. The wave of neural activity rose for about a second and then sharply decreased just after the onset of finger movement.
Kornhuber and Deecke coined this rise in neural activity the Bereitschaftspotential, also called the pre-motor potential or readiness potential, because they interpreted the activity pattern as the brain readying itself for movement.
Images from the 1964 experiment show the Bereitschaftspotential prior to voluntary finger flexion (left) and one of the participants hooked up to an EEG (right). (photo from the original experimental setup used in Kornhuber and Deecke)
Photo source: Lüder Deecke, Nature
This momentous discovery was the beginning of controversy in neuroscience. Twenty years later, Dr. Benjamin Libet, a researcher at the University of California at San Francisco, argued that not only does the brain show signs of a decision before movement, but the brain becomes active before a person consciously decides to act. In a study published in Brain, Libet and colleagues repeated the original Bereitschaftspotential experiment but asked his participants to report exactly when they made a decision. Results showed that neural activity started to rise about 500 milliseconds before participants made an action, but that decisions to act were reported only 150 milliseconds beforehand.
“The brain evidently ‘decides’ to initiate the act” before a person is even aware that decision has taken place, Libet concluded. Thus, a neurological argument against free will was born.
Scientists, however, questioned Libet’s experimental design, including the precision of tools he used to measure brain activity. But even recent studies using modern fMRI technology replicated his findings over and over again. How could it be that our conscious awareness of a decision is simply a fleeting afterthought?
In the original 1967 Bereitschaftspotential experiments, the two German scientists used state-of-the-art computers to record brain activity. But the machine only worked after it detected a finger flexion. So, they recorded brain activity separately on tape and used an inventive technique called ‘reverse-averaging’ to play the tape recordings backward into the computer. Using this inventive data analysis method, Kornhuber and Deecke revealed the Bereitschaftspotential. In recent years, researchers began to realize that this method was key to properly define the readiness potential.
A modern take on decision-making: debunking a scientific argument against free will
In 2010, Dr. Aaron Schurger, a researcher at the National Institute of Health and Medical Research in Paris, had an epiphany. At any given time, neural activity or electrophysiological noise in the brain naturally fluctuates in slow tides, much like the surface of the ocean. This data is void of pattern; it is simply, noise. If someone lined up this noise at its peaks and reverse-averaged them in the manner of Kornhuber and Deecke’s original approach, the results would look exactly like rising trends, explained Schurger. There would be no purpose behind these apparent trends – they would represent a coincidence and not a meaningful signal.
“I thought, Wait a minute,” Schurger says. If he applied the same method to the spontaneous brain noise he studied, what shape would he get? “I looked at my screen, and I saw something that looked like the Bereitschaftspotential.”
Schurger and colleagues repeated a version of Libet’s experiment in a paper under review for publication at Proceedings of the National Academy of Sciences. To avoid cherry-picking brain noise, they included a control condition in which participants did not move at all. An artificial-intelligence program enabled them to find the moment when brain activity in the two conditions differed. If Libert were right, brain activity should have diverged at 500 milliseconds before the movement. But the algorithm spotted the difference just 150 milliseconds before the movement, precisely when people reported making decisions in Libet’s original experiment.
In other words, people’s subjective experience of a decision matched the actual moment their brains showed signs of making a decision. Libet’s findings were debunked.
When Schurger first proposed the neural-noise idea in 2012, the paper was largely ignored, but it was acknowledged by neuroscience. Schurger received awards for repealing a long-standing idea. “It showed the Bereitschaftspotential may not be what we thought it was. That maybe it’s in some sense artifactual, related to how we analyze our data,” says Uri Maoz, a computational neuroscientist at Chapman University.
Speculation about whether or not we have free will is rooted in philosophy - but, as Libet and Schurger have shown, neuroscience tools can reveal interesting insights into how our brains control decision-making, and perhaps how humans experience free will.